The present invention disclosed, in Japanese Patent Application No. 90649/1981, that a receiver including such a SAW amplifier is excellent in sensitivity and in noise suppression because such an amplifier serves as a narrow band tracking filter also for reception of an FM stereophonic broadcasting, for example, having a frequency band much wider than the pass band of the SAW amplifier.
The circuit arrangement of the above-introduced application, however, involves the following drawbacks.
(1) Since the SAW amplifier has narrow band amplification characteristics, the group delay is large and greatly varies with frequency. Due to this, signal distortion increases as the frequency deviation of the received FM wave increases and as the modulation frequency of the wave becomes higher, thus disabling tracking and reception of the signal in the worst case.
(2) Due to the large group delay, signal separation is extremely worsened.
(3) Since two outputs of the SAW amplifier which are derived from acoustic waves propagating in the opposite directions are used, two circuit lines respectively including frequency converters and IF amplifiers must be provided, thus inviting a complication of circuit arrangements and an increase of the fabrication cost.
The above-mentioned drawbacks (1) and (2) will occur if a closed tracking loop as disclosed in the prior art application includes therein a SAW device having characteristics shown in FIG. 1. The Figure shows the amplification ratio (solid line ) and the group delay (dotted line) of the SAW amplifier with respect to the frequency. When the pass band width at -3 dB is 10 kHz, the group delay is 40.mu. sec at the center frequency and becomes smaller as it comes apart from the center frequency. Outside the pass band, the group delay represents a constant value (several .mu. sec) determined by the propagating velocity and distance of surface acoustic waves. Thus, the group velocity largely varies with frequency.